Evidence has accumulated to show that the hypoxic cells in solid tumors undergo reoxygenation following irradiation and become susceptible to subsequent irradiation, and that the extent of such reoxygenation of hypoxic cells may be the determining factor in the curability of tumors by radiotherapy. Although it has often been mentioned that vascular function and oxygen consumption might play an important role in the reoxygenation process, systematic investigation of the relationship among these in the irradiated tumors has not been done. In the present study, the sequential change in oxygenation of tumor cells and in the various aspects of vascular function as well as oxygen consumption following X-irradiation will be investigated in experimental tumors. Specifically, the functional intravascular volume and vascular permeability will be measured simultaneously by 51Cr-RBC and 125I-plasma protein, respectively. The blood flow will be quantitated using the clearance rate of 133Xe from tumor after interstitial injection. The vascular occlusion and the formation of arterio-venous shunt in the irradiated tumors will be investigated by autoradiographic study of the distribution of 51Cr-RBC in capillaries. Oxygen consumption rate in tumor cells will be determined with a sensitive oxygen analyzer. The proportion of hypoxic cells in the tumors will be computed from the cell survival curve obtained by colony formation in soft agar media in vitro.